The kink effect exhibited in field effect transistors (FETs) has long been studied, modeled and analyzed by engineering, research, and academic groups without producing an effective solution to eliminate it without introducing new problems. The kink effect derives its name from the sometimes sharp bend in the drain current-voltage (I-V) characteristics of the device. The I-V characteristics show undesirably high drain conductances that increase with increasing drain voltage. It has been shown that silicon-based field effect transistors as well as compound semiconductor-based field effect transistors experience the same kink effect.
Heterostructure field effect transistors are well suited for many high speed, high frequency applications. However, these heterostructure field effect transistors, are plagued by the same kink effect found in silicon field effect transistors. Quantum well heterostructure field effect transistors further aggravate the problem by confining the impact ionized holes. As a result, a substantially large gate-bias voltage is required to capture the holes out at the gate electrode.
In addition to the kink effect, a related effect caused by the impact ionized holes is a large gate leakage current. Some of the ionized holes are attracted to and collected by the negatively biased gate terminal and create a leakage current. Engineering and scientific literature have also documented other undesirable effects primarily caused by the impact ionized holes.
Accordingly, it is advantageous to provide a solution that removes the impact ionized holes to eliminate the kink effect, gate leakage, and other undesirable effects.
Accordingly, there is a need for field effect transistor structures that do not exhibit the kink effect and a method for making the same.
In accordance with the present invention, field effect devices are provided which eliminate or substantially reduces the disadvantages associated with prior field effect devices.
In one aspect of the invention, the field effect device consists of a substrate, a conducting backplane formed in the substrate, and a source and a drain disposed above the conductive backplane. A gate is insultatively disposed above the substrate between the source and drain, and a backgate contact is electrically coupled to the conducting backplane.
In another aspect of the invention, the method of making field effect devices includes forming a substrate, a conducting backplane disposed above the substrate, a source and drain disposed above the backplane, a gate contact insulatively disposed above the substrate between the source and drain, and a backgate contact coupled to the conducting backplane.
In yet another aspect of the invention, the method of operating a kink-free field effect device consists of applying bias voltages to a gate, source and drain of the field effect device. The method also consists of applying a negative bias voltage to a conductive backplane and attracting and collecting impact ionized holes generated near the valence band of the field effect device by the conductive backplane.
One technical advantage of the invention is its application to most if not all field effect transistor structures for neutralizing and collecting of impact ionized holes eliminate the kink effect and other undesirable characteristics. Another technical advantage of the invention is that the solution is directed at the source of the problemxe2x80x94the uncontrolled trapping and untrapping of impact ionized holes in the valence band. Yet another technical advantage of the present invention is its substantially total elimination of the kink effect by neutralizing and collecting substantially all of the impact ionized holes without the use of large bias voltages.
It may be seen that the conducting backplane is most effective in collecting the impact ionized holes if there is no barrier to hole-collection by the backplane in the valence band. In the present device the valence band barrier is removed by grading continuously (or in small steps) from the channel where the holes are generated (narrow Energy gap) to the lower confining barrier (wide Energy gap)hence eliminating the valence band barrier.
The backplane is less effective if the grading is absent. The grading does not deleteriously affect operation of the FET because the electron confinement is restored as the backplane is biased negatively, for hole-collection, with respect to the channel.